We have discovered that the win32k!xxxSendMenuSelect function has a bug which leads to the disclosure of uninitialized stack memory to user-mode clients, due to unused fields in a structure which ends up copied to userland.

The act of copying uninitialized kernel memory was originally detected under the following stack trace:

Where 00 denote bytes which are properly initialized, while ff indicate uninitialized values copied back to user-mode. As shown above, there are 12 bytes leaked at offsets 0x20-0x2b. We have determined that these bytes originally come from a smaller structure of size 0x1c, allocated in the stack frame of the win32k!xxxSendMenuSelect function. More specifically, we have found that the local structure is in fact of type MSG, and the uninitialized bytes correspond to the trailing "time" and "pt" fields:

Such a partially initialized structure is then passed down to win32k!_CallMsgFilter, and further down the call stack, before being eventually leaked to ring-3 through the user-mode callback. A proof-of-concept program is not provided for this issue, but it has been observed and confirmed at normal system runtime, and is quite evident in the code.

Repeatedly triggering the vulnerability could allow local authenticated attackers to defeat certain exploit mitigations (kernel ASLR) or read other secrets stored in the kernel address space.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public.